In a step-down DC-DC converter using a switching device and an inductor, it is required to reduce a so-called switching loss.
FIG. 4 illustrates a circuit configuration of a step-down DC-DC converter according to the background art.
As shown in FIG. 4, a DC-DC converter 80 of the background art has a switching device Q1, a smoothing inductor L1 and a rectifying diode D1. The DC-DC converter 80 steps down a direct current voltage that is supplied from a direct current power source Vin, and then outputs the same to a load R1. The switching device Q1 performs a switching operation in response to a signal (voltage Vc in FIG. 4) from a control circuit unit 82, thereby converting the direct current voltage into an alternating current voltage. The smoothing inductor L1 is provided for smoothing. The rectifying diode D1 rectifies alternating current power. An output capacitor C1 is connected in parallel with the load R1 and feeds power to the load R1.
In the DC-DC converter 80, when the switching device Q1 turns on, turn-on current Ion flows toward the smoothing inductor L1. When the switching device Q1 turns off, forward current (turn-off current) Ioff flows to the smoothing inductor L1 through the rectifying diode D1.
Here, when the switching device Q1 turns on, a reverse bias is applied to the rectifying diode D1 even though the current flowing in the rectifying diode D1 is cut off. During a reverse recovery time period, high recovery current Ir flows from a cathode of the rectifying diode D1 to an anode thereof. The recovery current Ir is superposed on the turn-on current Ion flowing in the switching device Q1. Thereby, the switching loss (turn-on loss) is caused.
JP-A-2007-185072 discloses a configuration of a DC-DC converter that reduces the recovery current Ir. In this DC-DC converter, a circuit, in which a secondary winding of an auxiliary transformer and a diode are connected in series, is connected to a direct current power supply, and a primary winding of the auxiliary transformer is connected to a switching device.